WRT54G v8 game lag

I experience severe lag when playing games. The game will usually be fine for about 15 minutes, and then the ping will skyrocket to ~1 second. This high ping will continue for a minute or so, then it seems to gradually reduce to about 200ms (still unacceptable.)

This is both on my PC with Call of Duty and on my Xbox360, playing Halo 3. If I plug either system directly into the modem, bypassing the router, there's no lag at all.

Recently I was doing the Linksys tech support dance. They told me to change the MTU to 1340. It didn't work; we tried 1450. Didn't fix it. 1400 didn't fix it either, so they sent me a replacement WRT54G (refurb I think). Of course, the same lag issue is still there with the new unit.

I don't know if this is some foul interaction between the router and the modem (it's an Ambit 60194e-A02).